1. Field of the Invention
The present invention relates to an apparatus in which a mechanical displacement proportional to a load change which is proportional to a quantity of a measured value such as pressure difference, pressure or liquid level is detected in terms of a variation of capacitance, and the capacitance variation is converted into a corresponding electric signal which is transmitted to a receiving apparatus. More particularly, the invention relates to a converting apparatus in which a pair of capacitors, the capacitances of which differentially change in response to a mechanical displacement, are used to produce the sum of and the difference between the capacitances of the capacitors. The sum and the difference are used to derive the amount of (capacitance difference)/(capacitance sum) as a ratio of the change of the capacitances, and then the ratio is converted into an electrical signal proportional thereto.
2. Prior Art
In a capacitor type displacement detector (a so-called .DELTA.C cell), a diaphragm for detecting a load change proportional, for example, to a pressure difference .DELTA.P is disposed at the middle position between electrodes, and a capacitance C1 between the diaphragm and one electrode and a capacitance C2 between the diaphragm and the other electrode are obtained as follows: ##EQU1## where .epsilon.:dielectric factor, A:electrode area, d:distance between electrode and diaphragm, and .DELTA.d:amount of diaphragm displacement. The sum and difference of these two capacitances are given as follows: ##EQU2## Therefore, the following equation is obtained. ##EQU3## As seen from the above equation, a pressure difference .DELTA.P may be detected by producing an electric signal proportional to (C1-C2)/(C1+C2).
In a known capacitor type displacement converter disclosed in Japanese Patent Application Publication No. 16,696/1978 published on June 2, 1978, a DC voltage Vi proportional to the difference between a pair of capacitances C1 and C2 and a DC voltage Vr proportional to the sum of these capacitances C1 and C2 are detected. Then, (C1-C2)/(C1+C2) is obtained by performing a division of Vi/Vr. In order to improve this displacement converter, Japanese Patent Application No. 154,321/1975 filed on Dec. 23, 1975 (Japanese Laid-Open Patent Application No. 76,950/1977 laid open on June 28, 1977) was proposed. In this displacement converter, the difference voltage Vi and the sum voltage Vr are converted into a pulse having a duty ratio corresponding to Vi/Vr. In this converter, however, the duty ratio changes, when the difference voltage Vi varies widely, resulting in a disadvantage in that there is an increase in the linearity error.